In recent years, there are a folding type wireless apparatus, a slide type wireless apparatus, two axes rotation type wireless apparatus or the like. Such the type of wireless apparatus includes two chassis. Each of the two chassis houses a circuit board.
In recent years, external appearance design of the wireless apparatus is considered to be important. For this reason, it is in progress to miniaturize and thin down the chassis of the wireless apparatus. Moreover, it is on the increase that a communication antenna is built-in within the chassis so that the external appearance design may not be damaged.
An example of the wireless apparatus which includes two chassis will be described in the following with reference to FIG. 10a and FIG. 10b. The wireless apparatus is the slide type (so-called slide type terminal). The wireless apparatus includes a liquid crystal display (hereinafter, referred to as LCD) side's chassis (LCD-side chassis) 100 and a key side's chassis (key-side chassis) 200. The LCD side's chassis 100 includes a LCD display unit 101. The key side's chassis 200 includes a handling unit 201. The LCD side's chassis 100 and the key side' chassis 200 are combined so that it is possible to change from a closed state as shown in FIG. 10b to an opened state shown in FIG. 10a, and vice versa.
FIG. 11 shows schematically a state (internal structure) in which the chassis is removed from the slide terminal shown in FIG. 10a. Referring to FIG. 11, the LCD side's chassis 100 houses a LCD side's circuit board (LCD-side circuit board) 102. The key side's chassis 200 houses a key side's circuit board (key-side circuit board) 202. An antenna element 204 is mounted to the key side's circuit board 202 in a state projecting from the key side's circuit board 202. Moreover, the key side's circuit board 202 includes an antenna feeding point 203 at which the antenna element 204 and an electric supply line are connected.
An antenna system, which is used in the wireless apparatus, is designed so that not only the antenna but also the LCD side's circuit board 102 and the key side's circuit board 202 can transmit (emit) a radio wave and receive a radio wave. Moreover, each of the circuit boards 102 and 202 includes a wireless circuit which is not shown in the Figure.
FIG. 12a shows a view of the structure shown in FIG. 11 from the key side's circuit board 202 side. FIG. 12b is a schematic side view of the structure shown in FIG. 12a. That is, FIG. 12b is a schematic diagram of the specific structure shown in FIG. 13, for the sake of easy understanding.
According to the slide terminal shown in FIGS. 10a to 13, a large metal member (that is, LCD side's circuit board 102 which is assumed as the ground) is arranged near the antenna. In this case, the metal member near the antenna often causes an influence on antenna characteristics. FIG. 14 is a Smith chart showing impedance characteristics of the antenna element 204 in the 2G frequency band. FIG. 15 shows frequency characteristics of return-loss in the antenna element 204. FIG. 14 and FIG. 15 will be described later in detail.
In particular, in the case of the slide type terminal or the like whose LCD side's circuit board 102 and the key side's circuit board 202 overlap each other, an electrical current flowing in the LCD side's circuit board 102 and an electrical current flowing in the key side's circuit board 202 are opposite in phase. The antiphase electrical currents cause the antenna characteristics to degrade. Due to the LCD side's circuit board 102 and the key side's circuit board 202 overlapping each other, frequency bandwidth, where the antenna element 204 and the wireless circuit are matched, becomes narrow. It is well known that the above mention is one of causes to degrade the antenna characteristics. Also in the case of a wireless apparatus, which includes two circuit boards, other than the slide type terminal, it is well known that the antenna characteristics are degraded as mentioned above due to overlapping two circuit boards.
In the case that it is possible to make the chassis of the wireless apparatus thicken, it is possible that a distance between the LCD side's circuit board and the key side's circuit board is expanded in a direction of thickness of the chassis. As a result, the antiphase electrical currents, which flow in the LCD side's circuit board and the key side's circuit board respectively, become reduced. Thus, it is possible to reduce mutual influence due to the circuit boards. However, there is a tendency to make the chassis thin as mentioned above, and consequently the distance between two circuit boards become close. For this reason, the antenna characteristics are degraded.
Patent Literature 1 proposes an art, as a related art, which can make bandwidth of the antenna characteristics broadband. According to the art, electrical current distribution in a flexible cable is controlled appropriately by a sleeve ground part. Namely, the art disclosed in Patent Literature 1 relates to the flexible cable which connects a LCD's side circuit board ground and a key's side circuit board ground each other. Specifically, the flexible cable includes a central ground part and the sleeve ground part which is arranged at both sides of the central ground part. The central ground part connects the LCD's side circuit board ground and the key's side circuit board ground. The sleeve ground part connects only with the LCD's side circuit board ground.